1. Field of the Invention
The present invention is directed to a device for eliminating ringing in filtered ECG signals, of the type having an analog-to digital converter for converting analog ECG input signals into a series of digital values, and a filter unit connected to the output of the analog-to digital converter. The present invention is also directed to a method for producing an allpass filter, for use in such a device, having a phase shift which is equal to twice the phase shift of an IIR filter having zero points which are situated on a unity circle in the Z-plane.
2. Description of the Prior Art
Different kinds of signal filtering are required for the analysis of ECG signals.
The ECG signal recorded between two electrodes placed on the body of a patient, for example, on the arms and legs, also contains a d.c. potential, in addition to the ECG signal. The d.c. potential may be much larger than the ECG signal. Low-frequency signals may also be present in the measured signal. The d.c. potential or the low-frequency components vary because of relative movements of the body of the patient and the electrode due, for example, to the patient's breathing movements.
Two-pass filtering is a known technique for removing these d.c. components and low-frequency components from the ECG signal. For this purpose, filtering with a linear phase is accomplished by conducting both forward and backward filtering of the signal employing a filter having a non-linear phase characteristic as described, for example, in United Kingdom Application 1 556 512, which describes such filtering using analog filters. A linear phase characteristic is desirable for the filtering, because phase distortion is then eliminated, which is the cause of minor distortions in the morphology of the ECG signal. A disadvantage of filtering with a linear phase characteristic, however, is that such filtering does not eliminate the problem of ringing arising in the filtered signal. The morphology of the waveforms or signal components produced by ringing depend on the transfer function of the filter, and will persist in the filtered signal. A so-called "FIR" (finite impulse response) filter can alternatively be used for providing a filter having a linear phase characteristic, but the steepness of the slope of the filter must be sacrificed if the filter is to be implemented in a device having limited memory or limited computational capacity.
The presence of high-frequency signals with small amplitudes on the microvolt level at the end of the QRS complex in post-infarction patients has been shown to be a good indication of the presence of an increased risk of future life-threatening ventricular arrhythmias, see European Heart Journal (1991), Vol. 12, pp. 473-480. High-pass filtering is needed in order to identify and analyze these late potentials. The creates the problem that the QRS signal itself induces ringing in the filtered signal, and this ringing masks any late potentials which may be present in the measured signal. Several proposals to solve this problem have been suggested. In an article by Link et al., "High-pass Filters for Detecting Late Potentials", Proc. Computers in Cardiology 1992, pp. 159-162, suggest the use of a non-recursive, monotonic, binomial, high-pass filter for detecting late potentials.
U.S. Pat. No. 4,422,459 proposes a technique for identifying the presence or absence of a time segment containing high-frequency signals in the latter part of a patient's QRS complex, and for measuring the magnitude of this segment. The measured analog ECG signals are converted into digital signals, and normal QRS complex signals are averaged over a hundred or more heartbeats in order to obtain a relatively noise-free complex. The averaged complex is then subjected to high-pass filtering in a fourth order Butterworth filter. The filtering is performed in the forward direction until the start of QRS+40 ms, and is then conducted in the backward direction from the end of the complex to the same point. The two filtered signals are then combined to form a resultant filtered signal, in which ringing is avoided immediately before and after the QRS, complex. The waveform of the QRS complex itself, however, is slightly altered, and therefore the avoidance of ringing is achieved only at the expense of producing an "erroneous" signal in the QRS complex itself. The resultant filtered signal is then subjected to measurements to ascertain the presence or absence of the aforementioned late potentials. The disadvantage of this technique is that the resultant filtered curve is discontinuous, and the measured version of the QRS complex obtained in this manner has an erroneous energy content as compared to the "true" QRS complex.
A system is disclosed in U.S. Pat. No. 4,458,691 for high-pass forward filtering of ECG signals for detecting late potentials. In this system, an adaptive high-pass filter for selective filtering of different segments of the QRS complex is employed to solve the problem of ringing. This technique also has several disadvantages. Ringing is not completely eliminated, and in order to reduce the ringing effect as much as possible, short FIR filters with a linear phase are used, which poorer frequency division.
Another technique for detecting late potentials by means of forward filtering of the ECG signal is disclosed in U.S. Pat. No. 4,458,692. In this method, the filter gain is regulated dependent on the magnitude of the input signal in an effort to solve the problem of ringing. The signal which controls the gain can be the output signal from a filter using the QRS signal as in input signal and, for example, the impact of the R-spike on the output signal, and thus on the ringing arising after the R-spike, can be limited if the gain in the filter is limited when the input signal rises above a defined level. This method also has the disadvantage of failing to eliminate all ringing, and a steep filter slope, which is otherwise desirable, must be sacrificed.
A method is also disclosed in U.S. Pat. No. 4,492,235 for detecting late potentials by means of forward filtration of the QRS signal, wherein adaptive high-pass filtering is also employed to address the problems of ringing.
Another method for forward filtration of late potentials is disclosed in U.S. Pat. No. 5,025,794, wherein sampled values of the QRS complex are subjected to both forward and backward filtering, with the filtered signals being added, and the summed signal is then smoothed to yield signal components corresponding to the late potential. The method disclosed in U.S. Pat. No. 5,025,794, however, is very complicated.